Method and system for correcting burst errors in communications networks, related network and computer-program product

ABSTRACT

The errors that may occur in transmitted numerical data on a channel affected by burst errors, are corrected via the operations of: ordering the numerical data in blocks each comprising a definite number of data packets; generating for each block a respective set of error-correction packets comprising a respective number of correction packets, the respective number identifying a level of redundancy for correcting the errors; and modifying dynamically the level of redundancy according to the characteristics of the bursts and of the correct-reception intervals between two bursts. Preferential application is on local networks, such as W-LANs for use in the domestic environments.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/397,330, filed Apr. 4, 2006, which is hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to techniques that enable correction ofburst errors in communications networks.

In a specific way, the invention relates to techniques for modifyingdynamically the level of redundancy for forward error correction (FEC)on the basis of the conditions of noise on the channel.

The invention has been developed with particular attention paid to itspossible use in wireless local-area networks (WLANs), but can be appliedto all communications networks that may be exposed to loss ofinformation (loss-prone networks).

2. Description of the Related Art

The growing number of applications that use transport protocols of anunreliable type, such as the UDP (unreliable datagram protocol), rendersnecessary the adoption of techniques for the recovery of the lostpackets to obtain error-resistant transmissions, above all in the casewhere the transmission occurs on error-prone communications networks,such as for example wireless networks.

An excessive packet-loss rate can very markedly reduce the quality ofthe received data. One of the techniques for recovery of lost packets(which preserves the quality of the data) uses the Reed-Solomon (RS)error-correction code. This technique envisages the generation of (n-k)redundant packets (FEC packets) for each block of k packets oftransmitted data. The FEC packets are transmitted together with the dataflow in order to protect the received data from losses. This type ofsolution enables protection of the data of up to a maximum of (n-k) lostpackets every n packets transmitted.

The disadvantages in the use of this technique regard optimal assignmentof the parameters n and k for error correction. The parameters can bechosen so as to maximize efficiency in terms of useful data using anerror correction that is precise to the point of compensating exactlythe level of loss and nothing else.

It is hence necessary to reach a balance between the amount ofinformation added by the error-correction code (which uses bandwidthsthat could be used for useful data and thus causes reduction in theefficiency of transmission of the data) and the minimum amount oferror-correction packets necessary on the basis of the conditions of thechannel (which can be useful to increase the efficiency of transmissionof the data, avoiding retransmissions).

Strictly connected to the above disadvantages is the problem of thedistribution of the recurrences (occurrences) of the lost packets. Ineffect, a loss model can have a more or less important effect on theresulting distortion at the receiver end, and hence accurate models forforecasting losses may represent a valid aid for minimizing theresulting distortion.

In particular, burst losses have a given recurrence on loss-pronenetworks, particularly wireless networks, and generally cannot be easilyor completely recovered. In the presence of burst errors, the techniquesbased upon the simple calculation of the mean packet error rate (PER)are those that afford the worst performance.

FEC packets generated by error-correction codes, such as theReed-Solomon code, on account of their capacity for recovering more thanone packets in a data block, are a good tool against burst distributionof losses.

In the presence of burst losses, the FEC packets can be more effectiveif they are used together with an interleaving technique.

Interleaving techniques enable random burst losses to be more sparsewithin the data flow and are based on the solution of transmitting thepackets not in their sequential order but according to a differentorder, defined by a pre-set law. An example of pre-defined order is theone shown in FIG. 1.

In particular, in FIG. 1 first all the packets “pck0” of the t blocksare transmitted in order starting from the packet “pck0” of block “0” upto the packet “pck0” of block “t-1”, then all the packets “pck1” aretransmitted always in the same order, and so on up to the packets“pck(k-1)”. After the k packets of all the t blocks have beentransmitted, the n FEC packets are transmitted once again in the sameorder.

The number of blocks designed to be interleaved is chosen taking intoaccount the need to render sparse the errors and respecting constraintsin terms of decoding delay, which are particularly important forreal-time applications (before starting the operation of decoding, allthe blocks involved in interleaving must have been received).

The problem can be reduced to the operation of finding the optimalvalues for the following parameters:

-   -   number k of data packets to be protected with the FEC packets;    -   number (n-k) of FEC packets that protect the k data packets        computed at the previous step; and    -   depth of interleaving (understood as the number of blocks that        are involved in interleaving).

The purpose of the above procedure is to enable maximization of theefficiency of transmission of the useful data and minimization of theresidual losses (losses of the data packets) at the receiver end.

Various known methods for providing error correction typically supply afixed number of FEC packets, without considering the expedience ofincreasing/decreasing the efficiency of transmission on the basis ofdetection of variations in the condition of congestion or noise on thechannel.

Other known solutions propose a dynamic modification of the percentageof packets that are introduced for correcting transmission errors. Themajority of these solutions do not prove, however, suitable formodelling and exploiting the measurement of the level of burstiness ofthe error model to improve protection in regard to errors. The meanpacket error rate by itself is not a good reference parameter forrepresenting the actual characteristics of the error model because, inthe presence of burst losses, the mean packet error rate is usually muchlower than the burst packet error rate.

The example of FIG. 2 shows a model that comprises 64 packets, in whicha “1” denotes a packet that has been received, a “0” denotes a lostpacket, and an “X” denotes a rejected packet.

The burst consists of the twelve packets designated in FIG. 2 by thereference “burst” that starts with a rejected packet X and ends with alost packet 0. The first correct-reception interval, designated by“gap”, begins at the start of the session, and the secondcorrect-reception interval terminates at the end of transmission.Half-way between the two correct-reception intervals there is preciselythe burst-error interval designated by “burst”.

The parameter Gmin is the minimum distance in the received packetsbetween two lost/rejected packets so that a loss can be considered asbelonging to a correct-reception interval.

For example, if the parameter Gmin is adjusted at the value 16, theburst density is 0.33 (4 lost or rejected packets divided by the totalnumber of 12 expected packets) and the density of the correct-receptioninterval is 0.04 (2 lost or rejected packets divided by the total numberof 52 expected packets). Consequently, the loss rate is 9.4%.

As has been said, FIG. 2 provides a real trace of the errors, where “1”means that the packet has been received and “0” that the packet has beenlost. The mean PER (calculated dividing the sum of the lost and/orrejected packets by the total number of expected packets) is 9.4%.Introduction of a FEC only on the basis of this parameter will rendernon-recoverable all the losses in the burst interval (i.e., 33% of theexpected packets). On the other hand, the FEC introduced in thecorrect-reception interval is useless.

Other known solutions propose dynamic methods for flexible adaptation ofthe error-correction capacity for recovering the lost packets andsimultaneously adapting the data rate in such a way as not to increasethe total data rate, applying a number of FEC packets. These methods areusually based upon the responses transmitted by the receiver to thetransmitter.

As regards specifically the patent literature, the document U.S. Pat.No. 5,699,365 describes a method and an apparatus for adaptive forwarderror correction in data-communication systems. This solution envisagesdynamic variation of the error-correction parameters on the basis of theconditions of the communication channel, as well as the level of noiseand the error rate.

In the method proposed in the document U.S. Pat. No. 5,699,365, data arereceived, which have a current degree of forward error correction, and achannel parameter is controlled, such as, for example, the bit errorrate or packet error rate. A threshold level is moreover defined for thechannel parameter, and the channel parameter controlled is compared withsaid threshold level. When the channel parameter does not fall within apre-set or adaptable range of variance of the threshold level, a newmodified parameter of forward error correction is selected, which has ahigher or lower degree of capacity for forward error correction. Thismodified error-correction parameter is transmitted on the channel. Thedevice that receives the modified error-correction parameter transmitsthe data encoded using the new modified error-correction parameter.

The document WO-A-2004/040831 describes a solution applied topacket-transmission networks. An adaptive scheme of error controlperformed at the application level is proposed, at the same timeenabling a maximum tolerated packet error rate to be respected.According to the scheme proposed, the amount of redundancy is adapted soas to afford a possibility of error correction enabling the maximumtolerated error rate to be respected. The maximum tolerated error rateis established by the particular application.

The document CA 2 430 218 describes an adaptive error-correctiontechnique based upon the noise bursts in which the rate of occurrence ofthese bursts is measured. The error-correction parameters are determinedusing statistics that describe the duration and period of the noisebursts. The occurrences, duration and period of the noise bursts aredetermined by the length of the error vector calculated during thedecoding process.

The document U.S. Pat. No. 5,181,207 describes a mechanism forcorrecting errors in a digital data stream encoded by the transmitterand transmitted on a communication channel to a receiver and decoded bythe receiver for recovering the digital data pre-encoded using a parityencoder. The position of the errors is determined by an operation ofcomparison, on the basis of which the receiver can generate a mask ofthe model of the errors in the received digital data stream. This maskof the error model is used in an preventive iterative error-correctionprocess for identifying the position of the burst errors contained inthe data recovered and for modifying the output of the host decoder ofthe receiver in a controllable way so as to eliminate the errorsidentified from the data originally decoded.

The document US-A-2004/015765 describes an adaptable and dynamic schemeof error correction for a communication channel. The method calculatesthe real bit error rate in order to make a comparison with a referencebit error rate. When a channel provides a performance better than therequired performance, the power of forward error correction can bereduced in order to provide a better efficiency. If the real bit errorrate calculated is higher than the reference bit error rate, then thepower of error correction is increased in an attempt to reduce the biterror rate. A feedback loop is used for continuous calculation of themodified bit error rates whilst the power of error correction isincreased or decreased.

The document U.S. Pat. No. 6,741,569 describes a subjectivequality-control system for multimedia-signal packet-transmission systemsthat evaluates the parameters of a statistical model that represents theprobability of the packet-transmission system that is in a condition oflow losses or in a condition of high losses and uses said evaluations ofthe parameters for predicting the subjective quality of the multimediasignal. The quality-control system contains a plurality ofquality-control functions located in the point of conversion betweenmultimedia signal and packet.

BRIEF SUMMARY OF THE INVENTION

From the foregoing description of the current situation, it emerges thatthere exists the need to define solutions capable of treating, in a moresatisfactory way, burst error correction in communications networks.

The object of the present invention is to meet the aforesaid need.

In accordance with the present invention, the above object is achievedby a method having the characteristics recalled in the claims thatfollow. The present invention also relates to a corresponding system, anetwork comprising said system, as well as a computer-program product,loadable into the memory of at least one computer and comprisingsoftware code portions for implementing the aforesaid method. As usedherein, the reference to such a “computer-program product” is to beunderstood as being equivalent to the reference to a medium that can beread by a computer and contains instructions for controlling a computersystem in order to co-ordinate execution of the method according to theinvention. The reference to “at least one computer” is aimed athighlighting the possibility for the present invention to be implementedin a distributed and/or modular way.

The annexed claims form an integral part of the disclosure of theinvention provided herein.

Basically, the solution described herein uses detailed information onthe description of the error model (see, for example, the document “RTPControl Protocol Extended Reports (RTCP XR)” Network Working Group RFC3611, available on line at the date of filing of the present applicationon the web site at the addresshttp<colon><slash><slash>www<dot>ietf<dot>org<slash>rfc<slash>rfc3611<dot>txt?number=3611(where “<colon>”, “<slash>” and “<dot>” represent correspondingsymbols)) and defines the method for using this information in order toderive the percentage of FEC packets and the interleaving factor.

In the currently preferred embodiment, the solution described herein isbased upon the information transmitted from the receiver to thetransmitter. The information includes the mean packet error rate (PER)but also some supplementary statistical information that aims atcharacterizing the loss model better in terms of length and recurrenceof the burst losses. The transmitter uses all the statisticalinformation for calculating the best level of protection to apply.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will now be described, by way of non-limiting example,with reference to the annexed figures of drawing, wherein:

FIGS. 1 and 2, referring to the prior art, have already been describedpreviously;

FIG. 3 shows an example of a domestic scenario of application of thesolution described herein; and

FIG. 4 shows an example of transmission on a communications network inthe framework of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For reasons of simplicity and as typical example of application of theproposed solution, the domestic scenario of continuous flow of packetdata shown in FIG. 3 is considered, in which a user or client 140 wishesto receive a video program (stream) from a server 100 through an accesspoint 120 and by means of a wireless card, for example of the 802.11xtype. The variations in conditions of the wireless connection in thistypical domestic environment, due to the effect of propagation orinterference of traffic generated by other devices present, may causethe loss of some packets (it is possible to consider another PC in thesame home that has access to the Internet via the same server andthrough the same access point).

A way for controlling the quality of the transmission and of theconnection is provided by the statistical information signalled by acontrol protocol, such as for example RTCP (Real-Time Transfer ControlProtocol), in the periodic responses that the receiver sends to thetransmitter. In particular, as for example in the solution describedherein, it is possible to consider more detailed metrics, such as theones defined in the document “RTP Control Protocol Extended Reports(RTCP XR)” Network Working Group RFC 3611. These metrics are to beconsidered supplementary with respect to the ones envisaged of thenormal RTCP report.

FIG. 4 shows an example of transmission, on a communications networkbased upon a real-time protocol (such as RTP), between a transmitter 200and a receiver 220 by means of a communication channel 240.

The transmission channel 240 is prone to loss of information. On thechannel 240 blocks of data (Block 0, Block 1 . . . ) are sent, each ofwhich contains the k data packets to be transmitted (pck0, pck1, . . .pck(k-1)) and the (n-k) error-correction packets (FEC0, FEC1, . . .FEC(n-k-1)). At pre-set intervals, the receiver 220 sends to thetransmitter 200 periodic responses 260 containing information andmetrics on the transmission by means of the extended reports (XRs).

The document RFC 3611 defines statistics capable of describing in a verydetailed way the losses that have occurred during a period ofobservation between two successive report blocks: the losses aredescribed not only in terms of mean PER, but also in terms of length ofthe burst, length of the correct-reception interval, and density (wherea burst is a period during which the rate of loss/rejection of packetsis large enough to cause of a considerable degradation of quality,whilst a correct-reception interval is a period during which there is arare occurrence lost/rejected packets, and hence the resulting qualityis generally very good).

The main idea of the solution described herein is the definition of amethod that uses the characteristics of the bursts and of thecorrect-reception interval as parameters for identifying optimal valuesfor the parameters k and (n-k) of forward error correction to maximizethe efficiency of transmission of the data and minimize the residuallosses.

The sequel of the present description makes express reference to someparameters appearing in RTP documents, such as:

-   -   “A Transport Protocol for Real-Time Applications”, Network        Working Group RFC 3550,        http<colon><slash><slash>www<dot>ietf<dot>org<slash>rfc<slash>rfc3550<dot>txt?number=3550;        and    -   “RTP Control Protocol Extended Reports (RTCP XR)”, Network        Working Group RFC 3611,        http<colon><slash><slash>www<dot>ietf<dot>org<slash>rfc<slash>rfc3611<dot>txt?number=3611.

The first of the aforesaid parameters is the PER. The packet error rateor the rate of loss of the packet is perhaps the simplest metrics thatcan be computed to evaluate the conditions of transmission. If theinterval between two reception reports is considered, the difference inthe cumulative number of lost packets gives the number of lost packetsduring the interval. The difference in the progressive numbers betweenthe last packets of two successive reception reports yields the numberof packets expected during the interval. The ratio of these two values(lost packets and expected packets) is the fraction of loss of packetsover the interval. This ratio is equal to the fraction of lost packetsonly if the two reception reports are successive; otherwise it is not.Furthermore, the duplicated packets will not be counted as receivedpackets, whereas the packets will be counted as lost if they arerejected on account of delayed arrival.

Another parameter is the so-called burst length. As used herein “burst”means a sequence of packets that has the following characteristics:

i) it starts with a lost or rejected packet;

ii) it does not contain any occurrence of Gmin consecutively receivedpackets (where Gmin is the minimum distance in received packets betweentwo lost/rejected packets so that a loss can be considered to belong toa correct-reception interval); iii) it concludes with a lost or rejectedpacket.

Another parameter is then represented by the correct-reception intervalor “gap”. The correct-reception interval is defined as a period with lowpacket losses and/or rejects (i.e., the period of time of correctreception between two bursts).

The bursts and the correct-reception intervals are characterized by adensity. The burst density is the fraction of data packets that havebeen lost or rejected during the burst period from the start ofreception. The density of the correct-reception interval is the fractionof the data packets that have been lost or rejected within thecorrect-reception intervals between successive bursts from the start ofreception.

The following parameters are moreover defined:

-   -   k as output par->data (number of data packets to be protected        with the FEC packets);    -   (n-k) as output_par->redundancy (number of FEC packets that        protect the k data packets);    -   output_par->rate as new rate at which the application must        encode the new data once the parameters output_par->data and        output_par->redundancy have been computed;    -   output_par->interleaving as interleaving depth;

xr->burst_length, xr->burst_density, xr->g_min e xr->gap_length, asmetrics signalled (in mean values) by the extended reports; and

input_par->startrate as index of initial encoding of the application.

The solution proposed herein enables determination of the optimal valuesfor the FEC parameters in the following way:

the number of data packets to be protected by means of the FEC packets(the k data in a block, output_par->data) is computed as the sum of themean burst length and of Gmin minus the mean number of effective lossesin a burst (errors_in_burst), then the number of FEC packets (theredundancy, output_par->redundancy) is adjusted at the same value as theparameter errors_in_burst.

The above operations can be translated into the following pseudo-code:

data = xr->burst_length + xr->g_min loss_rate = xr->burst_densityerrors_in_burst=xr->burst_length*xr->burst_density/100output_par->rate=input_par->startrate*(1-loss_rate/100) output_par->data= data − errors_in_burst output_par->redundancy = errors_in_burst

Calculation of a new value for the output_par->rate parameter isnecessary in order not to exceed the available bandwidth. Furthermore,the value of the length of the correct-reception interval plus thelength of the burst can be used for fixing the interleaving depth, asindicated in what follows:

output_par->interleaving=(xr->gap_length+xr->burst_length)/xr->burst_length;

The advantage provided by the solution described herein is representingby the fact that good (if not excellent) values are achieved for thenumber k of data packets in a block, for the number (n-k) of FECpackets, and for the number of packets to be interleaved, proceeding notby trial-and-error, as in the case of decisions based upon the packeterror rate (PER), but following upon application of a rule.

A confirmation of the advantages afforded by the solution describedherein is provided by simulations obtained using data streams thatpresent a loss model obtained from transmission on LAN wirelessnetworks. A transmission data stream (duration 125 sec) is considered,during which there has been detected a mean packet error rate ofapproximately 18.3%. The first test, the results of which are given inwhat follows, is an evaluation of the reduction of the losses (in termsof residual losses, i.e., of data packets lost) when (n-k) FEC packetsare applied to blocks each with k packets.

The value n is a fixed parameter, such as the interleaving depth, whilst(n-k) is computed dynamically with respect to the rate of losses or“loss_rate” (PER) signalled periodically in the response reports. Thenumber (n-k) of FEC packets is computed according to the followingrules:

-   -   if the loss rate is 0<loss_rate<3%, then no action is taken;

if the loss rate is 3%<=loss_rate<10%, then, in a block of n packets,10% are FEC packets;

if the loss rate is 10%<=loss_rate<20%, then, in a block of n packets,20% are FEC packets;

-   -   if the loss rate is 20%<=loss_rate<30%, then, in a block of n        packets, 30% are FEC packets;    -   if the loss rate is 30%<=loss rate<40%, then, in a block of n        packets, 40% are FEC packets; and so forth.

The duration of the period monitored, starting from a value of 500 ms,changes on the basis of the time necessary for the complete reception ofthe packets comprised in an interleaving block. The results presented inTable 1 show the average residual losses obtained applying said rulesand varying the interleaving parameter depth.

On account of constraints on the decoding delay (30 packets per blockand 30 blocks per interleaving depth means that there is to be expectedthe reception of 900 packets before start of decoding), as best valuemay be considered the value of the residual data losses in the last rowand column in Table 1 appearing below. It should be noted that thisvalue has been obtained after a given number of attempts.

TABLE 1 Loss Data- rate block Redundancy Interleaving Residual Length n(%) length (k) (%) depth losses 125 sec 10 16.05 7.62 23.78 — 11.943 125sec 20 16.05 15.72 21.19 — 12.15 125 sec 30 16.05 23.79 20.43 — 11.73125 sec 30 16.05 23.78 20.48 2 10.46 125 sec 30 16.05 23.77 20.51 310.27 125 sec 30 16.05 23.69 20.61 5 9.528 125 sec 30 16.05 23.73 20.617 10.04 125 sec 30 16.05 23.7 20.6 9 9.63 125 sec 30 16.05 23.65 20.7111 9.1 125 sec 30 16.05 23.6 20.71 15 9.71 125 sec 30 16.05 23.57 20.6120 9.53 125 sec 30 16.05 23.4 20.75 30 8.38

Applying the solution described herein to assign in an optimal way thenumber of FEC packets and the interleaving parameters on the basis ofthe same percentage of redundancy, almost the same optimal value for theresidual losses can be obtained, as shown by the results given in Table3.

In order to render homogeneous the comparison between the data appearingin Table 1 and Table 3, the best case has been given again in Table 1(obtained for n=30 and interleaving parameter equal to 30) using aperiod for the response reports fixed at 2700 ms. The same value hasbeen used for the test in Table 3.

TABLE 2 Period Redundancy Residual Length monitored Loss rate (%) (%)losses (%) 90 sec 2700 ms 18.29 19.52 7.92

As may be noted from Table 3, in the case where the number of FECpackets and the interleaving parameter are dynamic and calculated asproposed in this patent, values of residual losses very close to thoseof the optimal case given in Table 2 are obtained.

TABLE 3 Period Redundancy Residual Length monitored Loss rate (%) (%)losses (%) 90 sec 2700 ms 18.29 19.61 8.54

In conclusion, the experimental data confirm that the use of the metricprovided by RTCP XR (in particular, as regards the density of the burstsand the length of the correct-reception intervals) and the adoption ofthis method is a good solution in order to arrive, by means of a rule(and hence not by means of attempts), at the optimal values for theparameters k and (n-k) and at the value of interleaving depth formaximizing the efficiency of the data and minimizing the data losses.

Consequently, without prejudice the principle of the invention, thedetails of implementation and the embodiments may vary, evensignificantly, with respect to what is described and illustrated hereinpurely by way of non-limiting example, without thereby departing fromthe scope of the invention, as defined by the ensuing claims.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet are incorporated herein byreference, in their entirety.

1. A method for correcting burst errors in a transmission of data in acommunications network, the method comprising: under control of atransmitter device that transmits data blocks to a receiver device viathe communications network, the data blocks each including a definitenumber of respective data packets, transmitting one or more first datablocks to the receiver device via the communications network;dynamically determining a level of redundancy for correcting bursterrors in a transmission of one or more second data blocks to thereceiver device according to characteristics of bursts andcorrect-reception intervals of the transmitted one or more first datablocks; and for each of the one or more second data blocks, generating arespective set of error-correction packets comprising a respectivenumber of correction packets, said respective number based on thedynamically determined level of redundancy; and transmitting to thereceiver device via the communications network the respective datapackets and the respective set of error-correction packets.
 2. Themethod of claim 1, further comprising receiving from the receiver deviceinformation indicative of the characteristics of bursts andcorrect-reception intervals, and dynamically determining the level ofredundancy based on the received information.
 3. The method of claim 2,wherein the information is received from the receiver device in one ormore transmission reports generated by the receiving device.
 4. Themethod of claim 1, wherein the respective set of error-correctionpackets is a set of forward-error correction packets.
 5. The method ofclaim 1, further comprising identifying the bursts as sequences ofpackets that start with a lost packet or a rejected packet, do notcontain any occurrence of a given number of consecutively receivedpackets, and conclude with a lost packet or a rejected packet.
 6. Themethod of claim 5, further comprising identifying the given number asthe minimum distance in received packets between two lost or rejectedpackets so that a loss can be considered as belonging to acorrect-reception interval.
 7. The method of claim 1, wherein thedynamically determining the level of redundancy includes dynamicallydetermining the level of redundancy according to at least one parameterchosen between: a density of the bursts, defined as fraction of lost orrejected data packets during the bursts; and a density of thecorrect-reception intervals, defined as fraction of lost or rejecteddata packets within correct-reception intervals between successivebursts.
 8. The method of claim 1, wherein the dynamically determiningincludes dynamically determining the level of redundancy according to apacket error rate, defined as a ratio between a total number of lostpackets and a total number of expected packets for transmission in agiven time interval.
 9. The method according to claim 8, furthercomprising: receiving transmission reports of said packets; anddetecting a difference in a cumulative number of lost packets as anumber of lost packets during one of said time intervals.
 10. The methodof claim 8, further comprising: receiving transmission reports of saidpackets; and detecting a difference in progressive numbers between thelast packets of two successive reports as the number of expected packetsduring the interval.
 11. The method of claim 8, wherein duplicatedpackets are not counted as received packets.
 12. The method of claim 8,wherein a counting a packet that is rejected on account of delayedarrival as a lost packet.
 13. The method of claim 1, further comprisingsubjecting the respective data packets corresponding to the data blocksto interleaving.
 14. The method of claim 1, further comprising:identifying the bursts as sequences of packets that start with a lostpacket or a rejected packet, do not contain any occurrence of a givennumber of consecutively received packets, and conclude with a lostpacket or a rejected packet; and determining for each of the one or moresecond data blocks the definite number of respective data packets as asum of a mean length of the bursts and of the given number ofconsecutively received packets minus a mean number of losses of packetsin a burst, so that the respective number of correction packets isadjusted based on the mean number of losses of packets in a burst. 15.The method of claim 14, further comprising determining the respectivenumber of correction packets in a way proportional to a packet errorrate.
 16. The method of claim 15, further comprising determining therespective number of correction packets according to the followingrules: when the packet error rate is lower than a given threshold,error-correction packets are not added; when the packet error rate iscomprised between two percentage values A and B, with B>A, in a block ofn packets, B % error-correction packets are added.
 17. The method ofclaim 1, further comprising: identifying the bursts as sequences ofpackets that start with a lost packet or a rejected packet, do notcontain any occurrence of a given number of consecutively receivedpackets, conclude with a lost packet or a rejected packet; subjectingthe data packets corresponding to the data blocks to interleaving; anddetermining an interleaving depth as a sum of a length of thecorrect-reception interval and a burst length divided by the burstlength.
 18. A system for correcting burst errors in the transmission ofdata on a transmission channel affected by burst errors, the systemcomprising: ordering means for ordering the data in blocks eachcomprising a definite number of data packets; generating means forgenerating for each of the blocks a respective set of error-correctionpackets comprising a respective number of correction packets, therespective number identifying a level of redundancy for correcting theburst errors; and modifying means for dynamically modifying the level ofredundancy according to characteristics of the burst errors and ofcorrect-reception intervals between two of the burst errors.
 19. Thesystem of claim 18, wherein the modifying means include means fordynamically modifying the level of redundancy according to at least oneparameter chosen between: a density of the burst errors, defined asfraction of lost or rejected data packets during the burst errors; and adensity of the correct-reception intervals, defined as fraction of lostor rejected data packets within correct-reception intervals betweensuccessive burst errors.
 20. The system of claim 18, wherein themodifying means include means for dynamically modifying the level ofredundancy according to a packet error rate, defined as a ratio betweena total number of lost packets and a total number of expected packetsfor transmission in a given time interval.
 21. The system of claim 20,further comprising: further generating means for generating, at timeintervals, transmission reports of the packets; and means for detectinga difference in a cumulative number of lost packets as a number of lostpackets during one of the time intervals.
 22. The system of claim 20,further comprising: further generating means for generating, at timeintervals, transmission reports of the packets; and means for detectinga difference in progressive numbers between the last packets of twosuccessive reports as the number of expected packets during theinterval.
 23. A communications network, comprising: at least one channelfor transmission of data affected by burst errors; and a system forcorrecting the burst errors, the system including: ordering means forordering the data in blocks each comprising a definite number of datapackets; generating means for generating for each of the blocks arespective set of error-correction packets comprising a respectivenumber of correction packets, the respective number identifying a levelof redundancy for correcting the errors; and modifying means fordynamically modifying the level of redundancy according tocharacteristics of the burst errors and of correct-reception intervalsbetween two of the burst errors.
 24. The network of claim 23, whereinthe network is a wireless local-area network.
 25. The network of claim23, wherein the modifying means include means for dynamically modifyingthe level of redundancy according to at least one parameter chosenbetween: a density of the burst errors, defined as fraction of lost orrejected data packets during the burst errors; and a density of thecorrect-reception intervals, defined as fraction of lost or rejecteddata packets within correct-reception intervals between successive bursterrors.
 26. The network of claim 23, wherein the modifying means includemeans for dynamically modifying the level of redundancy according to apacket error rate, defined as a ratio between a total number of lostpackets and a total number of expected packets for transmission in agiven time interval.
 27. The network of claim 26, further comprising:further generating means for generating, at time intervals, transmissionreports of the packets; and means for detecting a difference in acumulative number of lost packets as a number of lost packets during oneof the time intervals.
 28. The network of claim 26, further comprising:further generating means for generating, at time intervals, transmissionreports of the packets; and means for detecting a difference inprogressive numbers between the last packets of two successive reportsas the number of expected packets during the interval.
 29. Acomputer-readable medium having contents that cause a computing deviceto correct burst errors in a transmission of data according to a methodcomprising the operations of: ordering the data in blocks eachcomprising a definite number of data packets; generating for each of theblocks a respective set of error-correction packets comprising arespective number of correction packets, the respective numberidentifying a level of redundancy for correcting the errors; anddynamically modifying the level of redundancy according tocharacteristics of the burst errors and of correct-reception intervalsbetween two of the burst errors.
 30. The computer-readable medium ofclaim 29, further comprising generating the set of error-correctionpackets as set of forward-error correction packets.
 31. Thecomputer-readable medium of claim 29, wherein the modifying operationincludes dynamically modifying the level of redundancy according to atleast one parameter chosen between: a density of the burst errors,defined as fraction of lost or rejected data packets during the bursterrors; and a density of the correct-reception intervals, defined asfraction of lost or rejected data packets within correct-receptionintervals between successive burst errors.
 32. The computer-readablemedium of claim 29, wherein the modifying operation includes dynamicallymodifying the level of redundancy according to a packet error rate,defined as a ratio between a total number of lost packets and a totalnumber of expected packets for transmission in a given time interval.33. The computer-readable medium of claim 32, wherein the method furthercomprises: generating, at time intervals, transmission reports of thepackets; and detecting a difference in a cumulative number of lostpackets as a number of lost packets during one of the time intervals.34. The computer-readable medium of claim 32, wherein the method furthercomprises: generating, at time intervals, transmission reports of thepackets; and detecting a difference in progressive numbers between thelast packets of two successive reports as the number of expected packetsduring the interval.
 35. The computer-readable medium of claim 29,wherein the method further comprises: identifying the burst errors assequences of packets having the following characteristics: i) they startwith a lost packet or rejected packet; ii) they do not contain anyoccurrence of a given number of consecutively received packets; iii)they conclude with a lost packet or rejected packet; and determining thedefinite number of data packets as a sum of a mean length of the bursterrors and of the given number of consecutively received packets minus amean number of losses of packets in a burst, so that the respectivenumber of correction packets is adjusted based on the mean number oflosses of packets in a burst.
 36. The computer-readable medium of claim35, wherein the method further comprises determining the respectivenumber of correction packets in a way proportional to a packet errorrate.
 37. The computer-readable medium of claim 36, further comprisingdetermining the respective number of correction packets according to thefollowing rules: when the packet error rate is lower than a giventhreshold, error-correction packets are not added; when the packet errorrate is comprised between two percentage values A and B, with B>A, in ablock of n packets, B % error-correction packets are added.